1. Field of the Invention
This invention relates to the structure of an avalanche photodiode used for optical communication, which has features such as a high response speed on the order of Gbps, ease of production, and also high reliability.
2. Background Art
In subscribers optical communication network system in the next generation, an avalanche photodiode (APD) is required, which has a response speed on a order of Gbps and which can be produced at a low cost.
It is necessary for such an element to provide characteristic features such as a simple structure, a mass production ability and a reduction in cost, capability of hybrid mounting on a planar lightwave circuit (PLC) or the like by passive alignment, and also high reliability.
An APD, provided with a buried waveguide structure shown in FIG. 5 is disclosed in Japanese Patent Application, First Publication No. Hei 4-286168 as one of the conventional examples to attain such an objective. Referring to FIG. 5, the reference numeral 51 denotes an n.sup.+ -InP substrate, 52 denotes an n.sup.- -InP optical waveguide layer, 53 a ridge-type optical waveguide area, 54 a matching layer, 55 a InGaAs light absorbing layer, 56 an n.sup.+ -InP multiplication layer, 57 a p+ diffusion layer, 58 aguard-ring, 59 an InP buried layer, 510 a P-side electrode, and 511 denotes an N-side electrode. This conventional element is constructed by re-growing the active portion after being buried in InP 59 and also by integrating a passive semiconductor light waveguide 53.
Another example is an APD with a mesa-type surface light-incidence structure as shown in FIG. 6 (Shingakukai, Sougou Taikai, 1998, C-3-11). Referring to FIG. 6, the reference numeral 61 denotes an n-type InP substrate, 62 denotes an n-type InAlAs buffer layer, 63 a super lattice multiplication layer, 64 a p-type InP electric field buffer layer, 65 a p.sup.- -type InGaAs light absorption layer, 66 a p-type InP cap layer, 67 a p.sup.+ -type InGaAs contact layer, 68 a light receiving area, 69 a passivation layer, 610 a P-side electrode, 611 an N-side electrode, and 612 denotes an AR coat.
Furthermore, the other example is a mesa-type waveguide structure APD shown in FIG. 7 which is disclosed in Japanese Patent Application, First Publication No. Hei 6-237009. Referring to FIG. 7, the reference numeral 71 denotes an n-type InP substrate, 72 denotes an n-type InAlAs buffer layer, 73 a superlattice multiplication layer, 74 a p-type InAlGaAs field buffer layer, 75 a p.sup.- -type InGaAs light absorption layer, 76 a p-type InGaAs buffer layer, 77 a p-type InALAs cap layer, 78 a p-type InGaAs contact layer, 79 a polyimide passivation film, 710 a p-electrode, and 711 an n-electrode.
In the mesa-type APDs shown in FIGS. 6 and 7, the simple structure is adopted by directly coating a polyimide film as a surface protective film 69 and 79 on the active portion of the element formed by mesa-etching.
Still another example is a back-illumination structure planar-type APD shown in FIG. 8 reported in IEEE, Photonics Technology Letters, vol. 8, pp. 827-829, 1996. Referring to FIG. 8, the reference numeral 81 denotes an SIInP substrate, 82 denotes a p.sup.+ -type buffer layer, 83 a p.sup.- -type InGaAS light absorption layer, 84 a p-type InP electric field buffer layer, 85 a non-dope InAlAs/InAlGaAs superlattice multiplication layer, 86 an n.sup.+ -type InAlAs cap layer, 87 an n.sup.+ -type InGaAs contact layer, 88 an annular-type isolation trench, 89 a region converted into p-type region, 810 a guard ring, 813 a p-electrode, 811 an n-electrode, 814 a passivation film, and 815 an AR coat. In this case, since this element has a depleted light absorbing layer 83 which is in an order of thicker than 1 .mu.m, it is necessary for from the element structure to provide a guard ring 810.
In contrast, a conventional example shown in FIG. 9 is a pn photodiode which does not have the avalanche multiplication function and which has been disclosed in Japanese Patent Application, First Publication No. 275224, and in Shingaku Giho LQE 97-120 (1997). Referring to FIG. 9, the reference numeral 91 denotes a p-type light absorbing layer, 92 denotes an n-type electrode layer, 93 a carrier transit layer, 94 a p-type carrier blocking layer, 95 an anode electrode, 96 a cathode electrode, 97 a semiconductor substrate, 98 an n-type cliff layer, 99 an i-type setback layer, 910 a p-type contact layer. This is a mesa-type structured element developed for the purpose of obtaining a super-high speed response (40 to 160 GHz) and a high saturated output (.about.1V). This conventional example seems to have the same structure as that of the APD of the present invention, but this conventional example differs from the present invention in various points. The differences will be described below.
The conventional example shown in FIG. 5 has drawbacks in that the manufacturing process is complicated and the yield is generally low. The conventional APDs shown in FIGS. 6 and 7 have problems in that, since the mesa edge surface (particularly, since the mesa edge surface of the InGaAs light absorption layer) and the surface protective layer are not sufficiently stable, it is difficult to obtain high reliability. The conventional example shown in FIG. 8 has a problem in that its manufacturing process (especially, the manufacturing process of the guard ring) is quite complicated.
There is a problem in that there is no field buffer layer in between the carrier transit layer 93 and the p-type light absorbing layer 91, as seen when the operation of the conventional APD shown in FIG. 9 is considered. Thus, since the only layer to suppress the electric field of the light absorbing layer 91 below the generation limit of a tunnel dark current is the p-type light absorbing layer 91 with a narrow band-gap at the p-side, a problem will arises, when it is operated as an APD, in that the dark current will increase because of the field rise in the light absorbing layer 91 which is in contact with the carrier transit layer 93. Furthermore, a method for providing a high speed response to this conventional APD is disclosed by a reference. That is, as disclosed by the reference, Shingaku Gihou LQE 97-120 (1997), since the p-type light absorbing layer 91 of this conventional example is set not to be depleted at a biased condition, in order to obtain a high speed response, it is necessary for photo-excited carriers (electrons) generated at the light absorbing layer 91 to overcome a hetero-barrier caused by the discontinuity of bands between the p-type light absorbing layer 91 and the carrier transit layer 93. For this purpose, in the light absorbing layer, a very thin region (10 nm) 99 in contact with the carrier transit layer 93 is depleted as a high purity i-type layer, and in the carrier transit layer, a very thin region (10 nm) 98 in contact with the light absorbing layer is doped to a high concentration to form a delta-shaped n-type as the cliff layer. When such a layer structure is formed, and especially when the n-type high-concentration cliff layer is present as an indispensable layer, the dark current inevitably in the element grows in the light absorbing layer before applying the high electric field which is so high as to generate an avalanche multiplication. That is, with this conventional element structure, it is not substantially possible to construct an element which functions as an APD.
The present invention was made to solve the above problems, and it is therefore an object of the present invention to provide an APD having a high speed response on the order of Gbps, and having a simple and reliable structure such that the APD of the present invention can be used to form a novel subscriber optical communication network.